The proliferation of portable devices, remote sensing and Internet of Things (IoT) continues to increase. Portable devices, remote sensors, and many other devices rely on energy stored in batteries which need regular charging or replacement.
Electrical energy stored in the batteries come predominantly from wired sources. Conventional wireless power transfer relies on magnetic inductive effect between two coils placed in close proximity of one another. To increase its efficiency, the coil size is selected to be less than the wavelength of the radiated electromagnetic wave. The transferred power diminishes strongly as the distance between the source and the charging device is increased.
Another technique for wirelessly powering a device is to transmit RF signals from a multitude of RF sources to the device. To improve efficiency, the phases of the RF signals should add constructively at the device. To achieve this, in one conventional system the device to be charged transmits a pilot signal. Each transmitter records the phase of the received pilot signal and calculates a corresponding conjugated phase which it then uses to transmit an RF signal. The receiver must include circuitry to generate a pilot signal that is frequency matched to that of the transmitter. The transmitter also requires at least one receiver per transmit antenna thus further adding to the complexity of the system. Accordingly, such systems require relatively high complexity in both the transmitter as well as the receiver.
Another technique in setting the correct phase for each source in the transmitter is described in commonly assigned U.S. application Ser. No. 14/830,692. In accordance with this technique, the amount of the power received at the device is fed back to the transmitter array to adjust the phase of each RF source in the array so as to maximize the power received at the device.